Page 137 - Intro to Space Sciences Spacecraft Applications

’ 0 1 0 0 1 1 0 1 . . DIGITAL STRING
Introduction to Space Sciences and Spacecraft Applications
124
RETURN
b & & TO (RTz) ZERO

TO ZERO
( NRZ )
-V k=f-= 5th 4tb 5tb 6tb 7tb 8tb NoN-RETURN
2tb
tb
Figure 5-1 0. Digital baseband signals. The “ones” and “zeros” of digital
baseband signals are usually represented by voltage levels held for a
specified period of time.

The time that the specific voltage is held to represent each bit is called
the bit period (tb), as indicated in the figure. It is desirable to represent
each bit as long as possible, as this would allow a receiver more time to
recognize each bit as it is received. But for real-time (continuous) com-
munications, the number of bits representing each sample must be trans-
mitted before the next sample bit stream comes up. To ensure this, the bit
period must be less than or equal to the sample period divided by the num-
ber of bits per sample:
t
tb = 5 2 (sec/bit) (5 - 17)
n

The pattern of this voltage output now represents the information to be
transmitted and will be used as the baseband signal to modulate the carri-
er wave in the methods described next.

Digital Modulation Methods

The digital baseband signals that are the product of A/D conversion are
used in methods similar to those discussed earlier in analog modulation
schemes to impress information onto a carrier wave for transmission
between stations. It was mentioned earlier that analog baseband signals
were limited in frequency to about 10% of the carrier wave, and we find
that digital signals have similar limits on the number of bits that a carrier
wave can transport. We can simply consider a digital baseband signal as a
continuous waveform with a period equal to two bit periods, as depicted
in Figure 5-1 1 which shows the first few bits of a NRZ waveform.